One typical gas turbine engine includes multiple, nested coaxial spools. A first turbine is mounted to a first spool and a second turbine is mounted to a second spool that rotates at a higher speed than the first spool. In one example, the first turbine is a low pressure turbine and the second turbine is a high pressure turbine. A mid turbine frame is arranged axially between the low pressure turbine and the high pressure turbine.
One example mid turbine frame includes an inner turbine case that has to be configured to allow oil service lines to pass through the case. For example, the inner turbine case includes several large holes cut through the case such that oil service lines can pass through the holes. The inner turbine case also includes certain geometric features that balance stiffness in a circumferential direction. At circumferential locations where there are no service lines, there are additional holes cut so that the inner turbine case stiffness is balanced. These balancing holes are needed for structural purposes; however, these holes open up a flow area between inner and outer cavities which is not desirable from a secondary flow perspective.